Ferroelastic lead germanate thin film and deposition method

ABSTRACT

A Pb 3 GeO 5  phase PGO thin film is provided. This film has ferroelastic properties that make it ideal for many microelectromechanical applications or as decoupling capacitors in high speed multichip modules. This PGO film is uniquely formed in a MOCVD process that permits a thin film, less than 1 mm, of material to be deposited. The process mixes Pd and germanium in a solvent. The solution is heated to form a precursor vapor which is decomposed. The method provides deposition temperatures and pressures. The as-deposited film is also annealed to enhanced the film&#39;s ferroelastic characteristics. A ferroelastic capacitor made from the present invention PGO film is also provided.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to the field of integratedcircuit (IC) fabrication and, more particularly, to a ferroelastic leadgermanate film and metal organic chemical vapor deposition (MOCVD)method for the above-mentioned thin film.

Ferroelastic films have attracted great interest in recent years becauseof their potential for use in applications such as high-energy storagecapacitors and high-strain actuators/transducers. More recently, withthe development of microelectronic devices, ferroelastic thin films havebeen explored for use in microactuators, microelectromechanical systems(MEMS), and as decoupling capacitors in high speed multichip modules(MCMs). It has been found that for charge storage applications,ferroelastics are superior to ferroelectrics because the stored chargecan be completely released due to absence of remanent polarization.Ferroelastic films are superior to films with linear dielectrics becauseof their high dielectric constant and high charge storage density.However, it is difficult to obtain ferroelastic thin films with squarehysteresis loops and zero remanent polarization.

The fabrication and characterization of ferroelastic lead germaniumoxide thin films (PGO), especially Pb₃GeO₅ are of current interest. Leadgermanate is a relative new materials. The PbO—GeO2 binary system hasbeen studied by Speranskaya (1959). R. R. Neurgaonkar et al. grew singlecrystal of ferroelastic Pb₃GeO₅ by the Czochralski technique (1974). Theferroelastic properties in this material were first discovered by R. R.Neurgaonkar et al. The dielectric and electric-optics properties ofsingle crystal and polycrystalline materials have been reported in theliterature. The ferroelastic Pb₃GeO₅ belongs to the monoclinic spacegroup P2 at room temperature. The crystals are ferroelastic, but show nophase transitions up to the melting point (738° C.). The interestingfeature of this material is that Pb₃GeO₅ has ferroelastic properties,which are suitable for microelectromechanical system (MEMS)applications.

Ferroelectric lead germanate (Pb₅Ge₃O₁₁) thin films have been made bythermal evaporation and flash evaporation (A. Mansingh et al., 1980), dcreactive sputtering (H. Schmitt et al., 1984), laser ablation (S. B.Krupanidhi et al., 1991 and C. J. Peng et al., 1992), and sol-geltechnique (J. J. Lee et al., 1992). However, ferroelastic Pb₃GeO₅ thinfilms made by MOCVD processes have not been reported.

The present invention PGO film has ferroelastic properties that areuseful in microelectromechanical system (MEMS) and high speed multichipmodule (MCM) applications. In co-pending patent application Ser. No.09/301,435, entitled “Multi-Phase Lead Germanate Film and DepositionMethod”, invented by Tingkai Li et al., filed on Apr. 28, 1999, attorneydocket No. SLA400, a second phase of Pb₃GeO₅ is added to the Pb₅Ge₃O₁₁,increasing grain sizes without an increase in c-axis orientation. Theresultant film has increased Pr values and dielectric constants, anddecreased Ec values. Such a film is useful in MEM, MCM, DRAM, and FeRAMapplications.

In co-pending patent application Ser. No. 09/301,420, entitled “C-AxisOriented Lead Germanate Film and Deposition Method for Same”, inventedby Tingkai Li et al., filed on Apr. 28, 1999, attorney docket No.SLA401, the Pb₅Ge₃O₁₁, film is crystallographically oriented in thec-axis. This film has smaller Pr and dielectric constant values, and isuseful in one transistor (1T) applications.

In co-pending patent application Ser. No. 09/302,272, entitled“Epitaxially Grown Lead Germanate Film and Deposition Method”, inventedby Tingkai Li et al., filed on Apr. 28, 1999, attorney docket No.SLA402, now U.S. Pat. No. 6,190,925, an epitaxial grown PGO film isdisclosed with extremely high c-axis orientation. As a result, high Prand Ec values, as well as lower dielectric constant, is obtained. Such afilm is useful in 1T, and one transistor/one capacitor (1T/1C) FeRAMapplications. The three above-mentioned co-pending patent applicationsare incorporated herein by reference.

It would be advantageous if a CVD process could be developed for thedeposition of ferroelastic PGO thin films.

It would be advantageous if a CVD process, offering the advantages ofexcellent film uniformity, compositional control, high film densities,high deposition rates, excellent step coverage, and commercialamenability, could be developed for PGO processes.

Accordingly, a method for forming a ferroelastic lead germanium oxide(PGO) film on an integrated circuit (IC) IC wafer has been provided.Typically, the wafer is at least partially covered with a conductiveelectrode material of Ir or Pt. The method comprises the steps of:

a) mixing [Pb(thd)₂] and [Ge(ETO)₄] to form a PGO mixture having a molarratio of about 3:1;

b) dissolving the mixture of Step a) with a solvent of tetrahydrofuran,isopropanol, and tetraglyme, having a molar ratio of about 8:2:1,respectively, to form a precursor solution having a concentration ofapproximately 0.1 to 0.5 moles of PGO mixture per liter of solvent;

c) introducing the precursor solution to a precursor vaporizer at a ratein the range of approximately 0.1 to 0.5 milliliters per minute (ml/mm),and heating the solution to create a precursor gas having a temperaturein the range of approximately 180 to 250 degrees C and a precursor vaporpressure in the range of approximately 30 to 50 torr (T);

c₁) mixing the precursor gas in the chamber with an argon gas shroudflow in the range of approximately 4000 to 6000 square cubic centimetersper minute (sccm), preheated to a temperature in the range ofapproximately 170 to 250 degrees C;

c₂) introducing an oxygen flow to the reactor in the range ofapproximately 1000 to 3000 sccm, whereby a lead-germanium oxide with ac-axis orientation is promoted;

d) heating the wafer chuck to a temperature in the range ofapproximately 500 to 650 degrees C, establishing a reactor chamberpressure in the range of approximately 5 to 10 T, and decomposing theprecursor gas on the IC wafer to form a PGO thin film with a thicknessof less than 1 millimeter (mm), including a first phase of Pb₃GeO₅,whereby the PGO film having ferroelastic properties is formed;

e) cooling the PGO film to approximately room temperature in an oxygenatmosphere; and

f) annealing the PGO film formed in Step d) in an atmosphere selectedfrom the group of oxygen and oxygen with Pb atmospheres, with the oxygenbeing introduced at a partial pressure greater than approximately 20%,whereby the ferroelastic properties of the PGO film are improved.

In some aspects of the invention a ferroelectric device is formed withthe PGO film, and further steps follow Step f) of:

g) forming a conductive electrode overlying the PGO film; and

h) annealing the PGO film in an atmosphere selected from the group ofoxygen and oxygen with Pb atmospheres, with the oxygen being introducedat a partial pressure greater than approximately 20%, whereby theinterface between the PGO film and the electrode formed in Step g), isimproved.

Typically, Steps f) and h) include using a rapid thermal annealing (RTA)process to anneal the PGO film, in which the temperatures are in therange of approximately 500 to 750 degrees C, for a duration in the rangeof approximately 10 to 30 minutes, and a thermal temperature ramp-up inthe range of approximately 10 to 200 degrees C per second. Alternately,furnace annealing is performed at temperatures between 500 and 600degrees for time durations of 30 minutes to 2 hours.

A lead germanium oxide (PGO) thin film having ferroelastic properties isalso provided. The PGO film comprises a first phase of Pb₃GeO₅, with thethickness of the first phase of Pb₃GeO₅ being less than approximately 1mm, whereby said Pb₃GeO₅ phase improves the ferroelastic properties ofthe PGO film. Typically, the first phase Pb₃GeO₅ has a grain size in therange of approximately 1 to 2 microns.

Also provided is a capacitor. The capacitor comprises a first conductiveelectrode, a PGO thin film including a first phase of Pb₃GeO₅ overlyingthe first electrode, and a second conductive electrode overlying the PGOfilm, whereby a PGO film capacitor is formed having ferroelasticproperties. The capacitor has a dielectric constant in the range ofapproximately 50 to 100, and a leakage current of 4×10⁻⁶ A/cm2 at 100kV/cm. The minimum polarization voltage is approximately 1 volt, and thesaturation voltage is less than approximately 5 volts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates steps in a method for forming a ferroelastic leadgermanium oxide (PGO) film.

FIG. 2 illustrates a completed present invention capacitor.

FIG. 3 is the X-ray pattern of Pb₅Ge₃O₁₁ films of the present inventiondeposited at 550° C.

FIG. 4 is a SEM micrograph of the present invention PGO film.

FIG. 5 illustrates maximum polarization (Pm) and switching fields (Es)of the present invention films.

FIG. 6 illustrates an I-V curves of the present invention PGO film.

FIG. 7 illustrates the dielectric constant of the present invention PGOfilms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The ferroelastic Pb₃GeO₅ thin films were prepared on Ir or Pt coated Siwafers by metalorganic vapor deposition (MOCVD) and RTP(Rapid ThermalProcess) annealing techniques. The films were specular and crack freeand showed completely crystallization between 500 and 650° C. Goodferroelastic properties were obtained for a 300 nm thick film with Ir orPt electrodes. Polarization can be generated at an applied voltage aslow as 1V, and almost complete saturation occurs at 5 V. Polarizationdisappears after removal of the applied voltage. The leakage currentsincreased with increasing applied voltage, and were found about 4×10⁻⁶A/cm² at 100 kV/cm. The dielectric constant showed the similar behaviorto the most ferroelastic materials, which dielectric constant changingwith respect to the applied voltage. The maximum dielectric constant wasabout 50-100. This high quality MOCVD Pb₃GeO₅ films can be used forapplications of microelectromechanical system (MEMS) and decouplingcapacitors in high speed multichip modules (MCMs).

The lead germanium oxide (PGO) thin film of the present invention hasferroelastic properties. These films comprise a first phase of Pb₃GeO₅.The thickness of said first phase of Pb₃GeO₅ is less than approximately1 mm, whereby said Pb₃GeO₅ phase improves the ferroelastic properties ofthe PGO film. Typically, the first phase Pb₃GeO₅ has a grain size in therange of approximately 1 to 2 microns.

An EMCORE oxide, or other similar MOCVD reactor with liquid deliverysystem was used for the growth of Pb₃GeO₅ films. Such a system is shownin FIG. 1 of co-pending patent application Ser. No. 09/301,435, entitled“Multi-Phase Lead Germanate Film and Deposition Method”, invented byTingkai Li et al. The Pb₃GeO₅ films were deposited on 6″ Pt or Ircovered Si wafers using MOCVD processes. The precursors for PGO thinfilms listed in Table 1.

TABLE 1 The properties of precursors for PGO thin films VaporDecomposition Pressure Temperature Precursors Formula (mm Hg) (° C.)Pb(thd)₂ Pb(C₁₁H₁₉O₂)₂ 180° C./0.05 325° C. Ge(ETO)₄ Ge(C₂H₅O)₄ b.p.185.5° C.

[Pb(thd)₂] and [Ge(ETO)₄] in a molar ratio of 3:1 were dissolved in amixed solvent of butyl ether (or tetrahydrofuran), isopropanol, andtetraglyme in the molar ratio of 8:2:1. The precursor solutions have aconcentration of 0.1-0.5 M/L of Pb₃GeO₅. The solution was injected intoa vaporizer (180-250° C.) by a pump at a rate of 0.1-0.5 ml/min to formprecursor gases. The precursor gases were brought into the reactor usinga preheated argon flow at 170-250° C. The deposition temperatures andpressure are 500-650° C. and 5-10 Torr, respectively. The shroud flow(Ar 4000-6000 sccm) with oxygen (1000-3000 sccm) was led into thereactor. After deposition, the Pb₃GeO₅ films were cooled down to roomtemperature in an oxygen atmosphere. The Pb₃GeO₅ films werepost-annealed before, and after, deposition of the top electrodes usingRTP method. The post-annealing step before deposition of top electrodesis defined herein as the first annealing, and the post-annealing afterdeposition of top electrodes is defined as the second annealing.

The basic composition, phase, and electrical properties of the someearly experimental Pb₃GeO₅ films have been measured. The compositions ofthe Pb₃GeO₅ films were analyzed by using energy dispersion X-rayanalysis (EDX). The phases of the films were identified using X-raydiffraction. The thickness and surface morphologies of the films onIr/Ti/SiO₂/Si substrates were investigated by Scanning ElectronicMicroscope (SEM). The leakage currents and dielectric constants of thefilms were measured using HP4155-6 precision semiconductor parameteranalyzer and Keithley 182 CV analyzer respectively. Other of the filmswere measured by a standardized RT66A tester.

The present invention PGO films were deposited at temperature about500-650° C. The as-deposited films were specular, crack-free, andadhered well to the substrates. These films also showed very smoothsurfaces as viewed by means of both optical microscopy and scanningelectron microscopy. The film growth rates were typically in the rangeof 2-5 nm/min.

FIG. 1 illustrates steps in a method for forming a ferroelastic leadgermanium oxide (PGO) film. Step 100 provides an integrated circuit (IC)wafer or film. Typically, a further step (not shown), precedes Step 102,of depositing a conductive electrode overlying the IC wafer. Theconductive electrode material is selected from the group consisting ofIr and Pt. Step 102 mixes [Pb(thd)₂] and [Ge(ETO)₄] to form a PGOmixture having a molar ratio in the range of approximately 2:1 to 4:1.Typically, the molar ratio is approximately 3:1, but the ratio isaltered in response to the temperature of the precursor vapor and thepresence of a partial Pb atmosphere in the reactor chamber.

Step 104 dissolves the mixture of Step 102 with a solvent oftetrahydrofuran, isopropanol, and tetraglyme to form a precursorsolution. In some aspects of the invention, Step 104 includes thesolvents tetrahydrofuran, isopropanol, and tetraglyme being in a molarratio of approximately 8:2:1, respectively. Typically, Steps 102 and 104are performed simultaneously, and Step 104 includes forming a precursorsolution having a concentration of approximately 0.1 to 0.5 moles of PGOmixture per liter of solvent. Tables 2 and 3 display precursor andsolvents that are alternately used with the present invention process.

TABLE 2 The properties of precursors for PGO films Appear- ance Decomp-at room Vapor osition temp- Moisture Pressure Temp. Precursor Formulaerature stability (mm Hg) (° C.) GeH₄ Ge₂H₆ Ge₃H₈ Ge(ETO)₄ GE(OC₂colorless sensitive 185° C. H₅)₄ liquid GeCl₄ (C₂H₅)₂- GeCl₂ Pb Pb(C6-white 230° C./ 325° C. Tetraphenyl H5)4 powder 0.05 Pb(TMHD)₂ Pb(C₁₁-white 180° C./ 325° C. H₁₉O₂)₂ powder 0.05 Pb(C₂H₅)₄

TABLE 3 The properties of solvents for PGO films Solvents FormulaBoiling Temp. (° C.) Tetrahydrofuran C₄H₈O 65-67° C. (THF) Iso-propanolC₃H₇OH  97° C. Tetraglyme C₁₀H₂₂O₅ 275° C. Xylene C₆H₄(CH₃)₂ 137-144° C.Toluene C₆H₅CH₃ 111° C. Butyl ether [CH₃(CH₂)₃]₂O 142-143° C. Butylacetate CH₃CO₂(CH₂)₃CH₃ 124-126° C. 2-Ethyl-1-hexanolCH₃(CH₂)₃CH(C₂H₆)CH₂ 183-186° C. OH

Step 106 heats the solution formed in Step 104 to create a precursorgas. Typically, Step 100 provides a precursor vaporizer auxiliary to thereactor, and Step 106 includes using the precursor vaporizer to heat theprecursor solution to a temperature in the range of approximately 180 to250 degrees C, whereby the precursor gas is formed. In some aspects ofthe invention, Step 100 provides a liquid pump. Then a further step isperformed, following Step 104, and preceding Step 106. Step 104 a (notshown) uses the liquid pump to introduce the precursor solution of Step104 to the precursor vaporizer in Step 106 at a rate in the range ofapproximately 0.1 to 0.5 milliliters per minute (ml/min).

Step 108 decomposes the precursor gas formed in Step 106 on the IC waferto form a PGO thin film, including a first phase of Pb₃GeO₅. Unlike abulk material, Step 108 typically includes forming a PGO film having afilm thickness of less than approximately 1 millimeter (mm). Step 110 isa product where the PGO film that is formed has ferroelastic properties.

In some aspects of the invention, Step 100 provides that the IC wafer islocated in a reactor chamber or vacuum chamber. Alternately, the ICwafer is introduced in a step (not shown) before Step 108. Regardless,Step 106 a mixes the precursor gas in the chamber with an argon gasshroud flow in the range of approximately 4000 to 6000 square cubiccentimeters per minute (sccm), preheated to a temperature in the rangeof approximately 170 to 250 degrees C. Then, Step 106 b introduces anoxygen flow to the reactor in the range of approximately 1000 to 3000sccm.

Further, Step 100 provides that the IC wafer is located on a wafer chuckin the reactor with a chamber pressure established to promote the flowof precursor and gases. Step 106 includes establishing a precursor vaporpressure in the range of approximately 30 to 50 torr (T). Step 108includes heating the wafer chuck to a temperature in the range ofapproximately 500 to 650 degrees C and establishing a reactor chamberpressure in the range of approximately 5 to 10 T. Alternately said, theoxygen partial pressure is greater than 10%, preferably in the range ofapproximately 20 to 50%.

Typically, a further step follows Step 108. Step 108 a cools the PGOfilm to approximately room temperature in an oxygen atmosphere. Step 112anneals the PGO film formed in Step 108 in an atmosphere selected fromthe group of oxygen and oxygen with Pb atmospheres, whereby theferroelastic properties of the PGO film are improved. Typically the Pbatmosphere is in the range of approximately 0 to 30%.

In some aspects of the invention a ferroelastic device is formed withthe PGO film of in Step 108. Then, further steps follow Step 112. Step114 forms a conductive electrode overlying the PGO film. Step 116anneals the PGO film in an atmosphere selected from the group of oxygenand oxygen with Pb atmosphere. The interface between the PGO film,formed in Step 108, and the electrode formed in Step 114, is improved.Typically, Steps 112 and 116 include the oxygen being introduced at apartial pressure greater than approximately 20%.

Steps 112 and 116 include using annealing methods selected from thegroup consisting of furnace annealing at a temperature in the range ofapproximately 500 to 600 degrees C for a duration of approximately 30minutes to 2 hours, and rapid thermal annealing using temperatures inthe range of approximately 500 to 750 degrees C. When RTA is used inSteps 112 and 116, the duration is in the range of approximately 10 to1800 seconds, and the thermal temperature ramp-up in the range ofapproximately 10 to 200 degrees C per second. In some aspects of theinvention, the first annealing step is a furnace anneal, and the secondanneal is an RTA anneal. Steps 108, 112, and 118 include the Pb₃GeO₅first phase having a grain size in the range of approximately 1 to 2microns.

FIG. 2 illustrates a completed present invention capacitor havingferroelastic properties. Capacitor 200 comprises a first conductiveelectrode 202, a PGO thin film 204 including a first phase of Pb₃GeO₅overlying first electrode 202, and a second conductive electrode 206overlying PGO film 204, whereby a PGO film capacitor is formed havingferroelastic properties.

Capacitor 200 has ferroelastic properties include a dielectric constantin the range of approximately 50 to 100, and a leakage current of 4×10⁻⁶A/cm2 at 100 kV/cm. The minimum polarization voltage is approximately 1volt and the saturation voltage is approximately 5 volts.

FIG. 3 is the X-ray pattern of Pb₃GeO₅ films of the present inventiondeposited at 550° C. The composition and X-ray analysis confirm theformation of polycrystalline Pb₃GeO₅ films.

FIG. 4 is a SEM micrograph of the present invention PGO film. Theaverage grain size of the films is about 1.5 μm. The thickness ismeasured about 300 nm. For the surface morphology, the film appears tohave uniformly distributed fine grains, appears to be crack-free underSEM examinations.

FIG. 5 illustrates maximum polarization (Pm) and switching fields (Es)of the present invention films. The as-deposited Pb₃GeO₅ films show goodferroelastic properties. After the RTP annealing at 550-600° C. for 0.5hour, the Pb₃GeO₅ films exhibited a symmetrical ferroelastic hysteresisloop with higher polarization (Pm) and lower switching field (Es).Polarization disappears after removal of the switching field, as shownin FIG. 5. Polarization appears even at very low switching voltage of1V, and increases as the switching voltage increases.

FIG. 6 illustrates an I-V curves of the present invention PGO film. Lowleakage current density is an important consideration formicroelectromechanical device applications. FIG. 6 shows the I-V curveof 300 nm thick MOCVD PGO films. Excellent I-V characteristics areobserved. The leakage current density of the Pb₃GeO₅ thin filmsincreases as the applied voltage is increased, and is about 4×10⁻⁶ A/cm²at 100KV/cm.

FIG. 7 illustrates the dielectric constant of the present invention PGOfilms. The dielectric constant is also another important issue formicroelectromechanical system (MEMS) and decoupling capacitors in highspeed multichip modules (MCMs). The dielectric constant of the Pb₃GeO₅thin films show behavior similar to the most ferroelastic materials,where the dielectric constant changes with applied voltage. The maximumdielectric constant of the Pb₃GeO₅ thin films is about 50-100. Note,FIGS. 3-7 illustrate the results of experimental films, and are notnecessarily optimum values.

A Pb₃GeO₅ phase PGO thin film is provided. This film has ferroelasticproperties that make it ideal for many microelectromechanical memorycell applications. This PGO film is uniquely formed in a MOCVD processthat permits a thin film, less than 1 mm, of material to be deposited. Aferroelastic capacitor and MOCVD deposition method for this PGO film arealso provided. Other embodiments and variations of the present inventionwill occur to those skilled in the art.

What is claimed is:
 1. A capacitor comprising: a first conductiveelectrode; a lead germanium oxide (PGO) film overlying said firstelectrode, said PGO film consisting essentially of Pb₃GeO₅ having agrain size in the range of approximately 1 to 2 microns; and a secondconductive electrode overlying said PGO film.
 2. A capacitor as in claim1 in which a dielectric constant of said PGO film is in the range ofapproximately 50 to
 100. 3. A capacitor as in claim 1 in which a leakagecurrent density of the capacitor is 4×10⁻⁶ A/cm2 at 100 kV/cm.
 4. Acapacitor as in claim 1 in which a minimum polarization voltage of thecapacitor is approximately 1 volt and a saturation voltage of thecapacitor is approximately 5 volts.
 5. A ferroelastic capacitor in anintegrated circuit device comprising: a substrate; a PGO film on saidsubstrate and consisting essentially of Pb₃GeO₅ having a rain size inthe range of approximately 1 to 2 microns; and a conductive electrodeoverlying said PGO film.
 6. A ferroelastic capacitor in an integratedcircuit device as in claim 5 in which said PGO film has a thickness ofless than approximately 1 mm.
 7. A PGO film capacitor for use inintegrated circuit devices consisting essentialiy of: Pb₃GeO₅, whereinsaid PGO film has a thickness of less than approximately 1 mm and a rainsize in the range of approximately 1 to 2 microns.
 8. In an integratedcircuit, a ferroelastic capacitor comprising: a first conductiveelectrode selected from the group consisting of Ir and Pt; a leadgermanium oxide (PGO) film overlying said first conductive electrode,said PGO film consisting essentially of Pb₃GeO₅ having a grain size inthe range of approximately 1 to 2 microns; and a second conductiveelectrode overlying said PGO film.
 9. A ferroelastic capacitor as inclaim 8 in which said PGO film has a thickness of less thanapproximately 1 mm.